1. Field of the Invention
This invention relates to pneumatic tires having a lug pattern, and more particularly to a pneumatic tire capable of effectively preventing the occurrence of uneven wear when the tire is used as a heavy duty tire for vehicles such as truck and bus.
2. Related Art Statement
In the conventionally known generic lug type tires, in order to overcome plysteer force generated due to stretch deformation of an outermost belt layer in a bias belt, lugs are frequently arranged in a slant posture oppositely arranged in a direction of extending cords of the outermost belt layer with respect to an equator of the tire.
In FIG. 5 is shown an embodiment of a prior art tread pattern in this type of the tire. This tire comprises plural lugs 33 in each side end part of a tread portion 31, which are defined by lug grooves 32 arranged in each side end part of the tread portion 31 and inclined in a certain direction with respect to an equator X--X of the tire.
When the lugs are arranged in a posture of upward to the right as shown in FIG. 5, cords of an outermost belt layer in a bias belt located inside the tread rubber are extended in a posture downward to the right as shown by a phantom line in FIG. 5, and consequently the occurrence of plysteer force is effectively prevented.
In the ground contact area of this tire, however, reaction forces F.sub.1, F.sub.2 result from the squeezing out of the tread rubber at edges 33a and 33b in circumferential direction of each lug 33 in directions perpendicular to the extending directions of these edges. The reaction forces F.sub.1 and F.sub.2 are divided into components of force F.sub.1c, F.sub.2c facing each other in the circumferential direction of the tire and components of force F.sub.1w, F.sub.2w directing inward and outward in the widthwise direction of the tire. On the other hand, when the tire is turned, since the tread portion 31 is subjected to a large external force inward in the widthwise direction of the tire, such an inward external force is effectively offset at the edge 33b of the lug 33 producing the outward component of force F.sub.2w in the widthwise direction of the tire, specifically the edge 33b forms an acute intersection angle with respect to a side edge 31a of the tread portion 31 among the edges 33a and 33b of the lug 33 in circumferential direction of the tire, the inward component of force F.sub.1w in the widthwise direction of the tire acts at the edge 33a opposing to the edge 33b in a direction of increasing the magnitude of inwardly external force, and consequently there is caused a problem of considerably wearing the edge 33a and the neighborhood thereof.